Large quantities of methyl methacrylate (MMA) are used for producing polymers and copolymers with other polymerizable compounds. The principal application, consuming approximately 80% of the MMA, is the manufacture of polymethyl methacrylate acrylic plastics (PMMA). Methyl methacrylate is also used for the production of co-polymers, such as methyl methacrylate-butadiene-styrene (MBS), used for instance as a modifier for PVC.
It is therefore highly desirable that methyl methacrylate can be produced by a process which results as simple as possible, is cost-effective and protects the environment.
Methyl methacrylate (MMA) is presently manufactured by several industrial methods, the principal one being the acetone cyanohydrin (ACH) route, using acetone and hydrogen cyanide as raw materials by way of the resultant ACH as main intermediate. The intermediate ACH is then converted with sulfuric acid to a sulfate ester of the methacrylamide, methanolysis of which gives ammonium bisulfate and MMA. Although widely used, the ACH route co-produces substantial amounts of ammonium sulfate, treatment of which incurs very high costs.
Other processes, which are not based on the afore-mentioned ACH route, are described in the relevant patent literature and are also well established on a production scale. Among the raw materials used in this context as starting materials are those based on C-4-compounds, for example isobutylene or tert-butanol, which are converted by way of a plurality of stages to the desired methacrylic acid derivatives.
In particular, Asahi Kasei has developed a two-step process for producing MMA, firstly through oxidation of isobutylene or tert-butanol at high temperatures with atmospheric oxygen in the gas phase on a heterogeneous catalyst to give methacrolein (MAL), followed by a second step with oxidative esterification reaction of MAL with methanol and oxygen in the presence of a catalyst. This process is described inter alia in the publications U.S. Pat. No. 5,969,178 and U.S. Pat. No. 7,012,039. Detailed information about this process is also provided in the article Trends and Future of Monomer-MMA Technologies, SUMITOMO KAGAKU 2004-II. In this publication, the direct oxidation process developed by Asahi is described in detail and in particular the disadvantages of the second step corresponding to the direct oxidative esterification step converting MAL to MMA. Indeed, the collection of MAL, recycling of excess methanol, separation of by-products and the like can be assumed to require a large amount of energy. It appears that there is a problem with the yield dropping when trying to raise productivity and reducing the excess methanol to reduce recycling. In general terms, the formation of by-products during the oxidative esterification, such as carbon dioxide, methyl formate, undesired carboxylic ester, requires complicated steps to isolate the final product MMA. For example, the starting material MAL, used in the oxidative esterification, is an extremely unstable compound which is likely to easily undergo a polymerization reaction. These side reactions in the oxidative esterification reactor are strongly to be avoided, because the polymerization of MAL results in clogging and deactivation of the active sites on the catalyst's surface, and causes a decrease in the catalyst's performance. Consequently, it is actually difficult to perform a long-term continuous running because the by-products, such as polymers, tend to accumulate disadvantageously.
This problem is pointed out in JP2004-345974 filed in the name of Asahi Kasei Chemicals Corporation. In said document, it is explained that the small hole of the oxygen diffuser is usually plugged up by polymer by-products, resulting in a bad supply and circulation of oxygen gas in the reactor. The solution proposed in said document is a continuous oxidative esterification process, wherein the temperature of the oxygen in the reactor has to be maintained between 0° C. and 80° C. in order to reduce clogging in the direct oxidative esterification reactor.
Another solution to this clogging problem in the reactor is provided in JP2004-345975, filed in the name of Asahi Kasei Chemicals Corporation. In said document, it is described a continuous oxidative esterification process of MAL to MMA with oxygen and methanol in the presence of a heterogeneous noble-metal-containing catalyst, wherein the polymer by-products are dissolved in methanol and subsequently removed from the reactor.
In U.S. Pat. No. 6,040,472 B1, it is explained that the process from Asahi is only commercially satisfactory when the reaction if performed under special conditions, namely, a reaction temperature as high as 60° C. or more, and a MAL feed concentration of the reaction system as high as 20 wt % or more.
Nevertheless, under these reaction conditions, the selectivity for MMA becomes low and the by-production of methyl formate due to the oxidation of methanol is sharply increased.
As described in U.S. Pat. No. 6,040,472 B1, in order to solve the afore-mentioned problems, Asahi developed a new Pd catalyst to be used in the one-step process for preparing MMA through oxidative esterification of MAL with methanol and oxygen, which allows working under these reaction conditions and still obtain a good yield and selectivity in the direct oxidative esterification of MAL to MMA.
However, there still exists the need to investigate further on possible alternative solutions in order to provide a technically and cost-effective MMA production process with high selectivity, which does not have the disadvantages of the afore-mentioned processes, like for example, by-product formation, too short run time periods for a continuous process because of clogging problems, low MAL conversion and low selectivity in the production of MMA.